Titanium alloys are widely used in the production of rotating blades for steam turbines. In particular, the rotating blades within low pressure steam turbines are exposed to high-speed collision of wet steam, causing erosion and abrasion due to the wet droplets in the steam. Titanium alloys exhibit a desirable level of resistance to the steam environment found within such turbines, where various treatments of the titanium alloy have been required to improve the service life of the rotating blades. For example, a Ti-6Al-4V titanium alloy, is an alpha-beta titanium alloy comprising a high strength material commonly used for turbine engine components consisting principally of about 6 percent aluminum, 4 percent vanadium, and the balance titanium and other constituents.
Typically, titanium alloy components for turbine engine applications are produced through a forging process, followed by a heat treatment process designed to assure adequate strength and ductility. Various processes have been proposed to produce improved characteristics of the material forming the final component. For example U.S. Pat. No. 5,032,189 describes a process of fabricating forged near alpha and alpha+beta titanium alloy components including forging an alloy billet at or above the beta-transus temperature, heating the forged component at a temperature approximately equal to the beta-transus temperature, cooling the component and annealing the component at a temperature approximately 10-20% below the beta-transus temperature for about 4 to 36 hours.
Another approach to improving the mechanical properties of titanium alloys is described in U.S. Pat. No. 4,898,624. A titanium alloy is described having the following composition: 5.5 to 6.75% Aluminum, 3.5 to 4.5% Vanadium, 0.15 to 0.2% Oxygen, 0.025 to 0.05% Nitrogen, ≦0.3% Iron, 0 to ≦0.08% Carbon, 0 to ≦0.0125% Hydrogen, 0 to ≦0.005% Yttrium, residual elements each 0 to ≦0.1% total 0 to ≦4%, and the remainder Ti. The alloy is prepared with heat treatment processes to produce a microstructure having nearly equiaxed primary alpha particles with platelets of secondary alpha in an aged beta matrix, where the fracture toughness (KIC) is about 45 ksi-in1/2.
Changes to the microstructure of the titanium alloy to improve the fracture toughness generally require a compromise in other material properties. Such a compromise typically includes a reduction in the yield strength and/or a reduction in the ductility of the material. Accordingly, it is desirable to provide an improved process for increasing the fracture toughness of a titanium alloy while maintaining or limiting the reduction of other properties such as yield strength.